The invention relates to a high-frequency system for an MR apparatus with a high-frequency coil configuration comprising a plurality of resonator elements, which coil arrangement is connected to a transmit unit, where a respective transmit channel of the transmit unit is assigned to the resonator elements.
The invention further relates to an MR apparatus with a high-frequency system of this kind.
In respect of the MR image generation, the localization of the nuclear magnetization takes place within the examination volume by means of time-variable, spatially-inhomogeneous magnetic fields (magnetic field gradients). To generate the image, the MR signal is recorded in the time domain as a voltage, which is induced under the influence of a suitable sequence of high-frequency pulses and gradient pulses, in the high-frequency coil arrangement enclosing the examination volume. The actual image reconstruction then takes place by Fourier transform of the time signals.
Part of the high-frequency system of the usual MR apparatuses is a transmit and receive coil, such as an integrated body coil, which is usable for the volume image generation. Separate surface coils or so-called phased-array coils may also be used in order to achieve an improvement in receive signal quality (improved signal-to-noise ratio, higher resolution). The body coils used both for excitation and for detection of MR signals are normally so-called birdcage coils. These comprise a plurality of conductor bars arranged around the examination volume and running parallel with the main field direction, which conductor bars are joined together via circulator conductors on the end faces of the coil. The resonance characteristic of body coils of this kind is determined by capacitor elements, by means of which the conductor elements are connected to a network.
The parallel use of a plurality of surface coils for receiving the MR signals from the examination volume is known from e.g. U.S. Pat. No. 6,323,648. In accordance with this document, a plurality of surface coils, arranged, in particular, in the area of the extremities of the patient under examination may be operated in parallel in order thereby to combine the MR signals detected to form an overall image. The advantage of this is that, owing to the restricted spatial sensitivity range of the surface coils, a large signal-to-noise ratio arises. The combination of surface coils for image generation is also known by the designation of SYNERGY.
Very recently, the move has been towards using high-frequency coil arrangements with a plurality of resonator elements, which are connected to the transmit unit of the MR apparatus, for the transmit mode also, wherein a transmit channel of the transmit unit is assigned to each of the individual resonator elements.
As a result of a separate transmit channel being assigned to each resonator element of the high-frequency coil arrangement in MR apparatuses of this kind, the field distribution in the examination volume is, advantageously, completely controllable. It is hereby possible to generate any conceivable current distribution in the high-frequency coil arrangement by the individual preselection of amplitude and phase on the individual transmit channels. The time characteristic of the HF feed may also be individually preselected differently on each transmit channel. The opportunity thereby exists e.g. of simulating the field distribution of a conventional birdcage coil in any resonance mode. The amplitude and the phase of each individual transmit channel may hereby be controlled by the software of the MR apparatus, which makes a direct, interactive control of the field distribution (RF shimming) possible. It is, for example, also conceivable to integrate a fully-automatic loop control of the HF field homogeneity into the image-generation sequence in order to compensate for variable influences on the field distribution, such as those from the different dielectric properties of the patient under examination.
If available, surface coils to which separate transmit channels are again assigned may also be part of the high-frequency coil arrangement of the high-frequency system in accordance with the invention, so that the surface coils may also be used in transmit mode for the variable generation of the high-frequency field in the examination volume.
A number of interesting application fields are opened up as a result of the opportunity presented by high-frequency systems of this kind to preselect as desired the spatial distribution of the high-frequency field in the examination volume. So, for example, gradients may be generated in the high-frequency field in different directions in space. As a result of the preselection of spatially-variable and time-variable high-frequency field patterns, a local coding which may be used for a rapid image generation is impressed on the nuclear magnetization distribution thereby excited (this is known as the Transmit-SENSE method). Also conceivable is a spatially selective pre-saturation of the nuclear magnetization in the examination volume.
The problem is that, with conventional MR apparatuses, the use of a high-frequency system of the type outlined above is possible only with a great deal of complexity. In principle, the plurality of transmit channels makes a corresponding number of high-frequency power amplifiers necessary. The transmit units of conventional MR apparatuses are normally equipped with a multi-stage high-frequency power amplifier (transmitting amplifier), which has only one channel, although it is capable of making high-frequency outputs available in the range of several kW. The use of a plurality of transmitting amplifiers of this kind to supply a corresponding number of transmit channels would, disadvantageously, be extremely cost-intensive, since the kW transmitting amplifiers used in MR apparatuses are extremely costly components. On the other hand, it would not be expedient to use instead a plurality of low-power transmitting amplifiers since, as a result, the flexibility and variability in the generation of the high-frequency field in the examination volume, obtained as a result of the multi-channel design of the high-frequency system, would be severely restricted. It is true that the power of a few kW, which, as mentioned above, can readily be provided by conventional MR transmitting amplifiers, is also adequate overall for high-frequency systems with a plurality of transmit channels. However, the problem is that it has to be possible, depending on the application, either to distribute the total power uniformly over all resonator elements of the high-frequency coil arrangement, or to supply the total power via a single transmit channel to only one of the resonator elements.